Mechanical strain and estrogen activate estrogen receptor alpha in bone cells

Bone cells) early responses to estrogen and mechanical strain were investig ated in the ROS 17/2.8 cell line. Immunoblotting with antiphosphorylated es trogen receptor alpha (ER-cu) antibody showed that when these cells Were ex posed for 10 minutes to estrogen (10(-8) M) or a single period of cyclic dy namic strain (peak 3400 mu epsilon, 1Hz, 600 cycles), there was an increase in the intensity of a 66-kDa band, indicating phosphorylation of ser(122) in the amino terminus of ER-alpha. Increased phosphorylation was detected w ithin 5 minutes of exposure to estrogen and 5 minutes after the end of the period of strain. Estrogen and strain also activated the mitogen-activated protein kinase (MAPK) family member extracellular regulated kinase-1 (ERK-1 ), Increases in ERK activation coincided with increased ER-alpha phosphoryl ation, Activation of ERK-1 and the phosphorylation of ER-alpha, by both est rogen and strain, were prevented by the MAP kinase kinase (MEK) inhibitor U 0126 and the protein kinase A (PKA) inhibitor (PKI), These data support pre vious suggestions that resident bone cells' early responses to strain and e strogen share a common pathway, which involves ER-alpha. This pathway also appears to involve PKA and ERK-mediated phosphorylation of ser(122) within the amino terminus of ER-alpha, Reduced availability of this pathway when e strogen levels are reduced could explain diminished effectiveness of mechan ically related control of bone architecture after the menopause.